I think it's worth noting that since Scala 2.12, the Either implementation provided by Scala's Standard Library is right biased. So for this particular case, Scalaz \/ is no longer needed and it could be replaced by the standard Either.

true .. in Scala 2.12, Either is right biased. But scalaz \/ offers lots of combinators including itself being a Monad. Note Scala standard library does not have a generic Monad. So, yeah in some places you can replace \/ with Either, but not all.